Short Dark Periods Research Articles

Flowering responses to vernalization and photoperiod in annual medics (Medicago spp.)

The three processes thought to control flowering times in annual medics (Medicago spp.) are a vernalization requirement, a long day requirement and a high temperature requirement. To examine the first two processes, seed of seven cultivars of six species was vernalized at 1�C for periods of up to 11 weeks, then grown to flowering under three photoperiods in a glasshouse. To study the third process, the time to flowering of selected treatments from this expcrirnent was compared with flowering data from plants grown in the field at a range of temperatures lower than in the glasshouse. Vernalization and photoperiod caused large shifts in flowering time but the effects varied widely among species. M. scutellata was almost insensitive to both factors but in M. rugosa acceleration of up to 91 days was caused by treatment. Vernalization and short dark periods were additive in accelerating flowering and largely able to substitute for each other. Species flowered almost simultaneously when given their most favourable conditions for flowering. High temperature accelerated flowering in all species studied. However, in species other than M. scutellata it was necessary for a vernalization requirement to be met before this effect was observed. A new finding was that the vernalization response in M. truncatula and M. littoralis was largely reversed after more than 7 weeks of vernalization. This suggests a previously undetected flowering mechanism in these species.

Influence de la durée de l'éclairement sur la croissance du thalle de quelques algues (trois chaetophorales et une rhodophycee)

Thallus growth of some algae as a function of the light period (fluorescent white light about 1000 lux) shows that continuous light, or better 18 h of light, are the most effective; however short (1 or 2 h) and long dark periods inhibit growth. Except for Caespitella pascheri, monochromatic irradiations of 6 or 12 h (in addition to the 18 or 12 h of white light) inhibit the growth of algae. On the contrary, after 23 h of white light, monochromatic radiations stimulate growth and the more effective wavelengths are: 443, 475, 575 and 675 nm. With incandescent light (rich in near far-red and poor in blue light), the growth is low.

Detection of Increments in Gaussian Noise by Rhesus Monkeys

Rhesus monkeys were trained to make a differential response on a given trial depending on the occurrence of a 100-msec increment (10-msec rise-fall), ΔI, in continuous Gaussian noise I. The task is analogous to the YES-NO task for humans. Banana-pellet reinforcement was made contingent on two correct trials in succession. Each incorrect trial resulted in a short period of darkness. Following stabilization of performance during acquisition, data were collected in blocks of 100 trials. Each daily session consisted of six blocks. Generation of ROC's resulted from variation of a priori signal probability. With a priori probability at 0.5, estimates of d′ were collected for different values of I and ΔI. A surface relating d′ to I and ΔI resulted. Although, in general, findings were comparable to those in humans, some differences were noted. Response-latency data were also collected. [This work was supported in part by grants from National Institutes of Health.]

Photosynthesis, Respiration and Post‐illumination Fixation of CO2 by Corn Leaves as Influenced by Light and Oxygen Concentration

AbstractThe effect of oxygen concentration on the rate of CO2‐uptake in continuous and intermittent light was studied as well as the CO2‐fixation during a short dark period after light was turned off. In addition the dark respiration and the CO2‐compensation point of attached and detached corn leaves were determined. Leaves of 4 to 22‐day old plants were used as experimental material. A closed circuit system of an infrared carbon dioxide analyzer was employed to measure the rate of CO2‐exchange. It was found that in an atmosphere consisting of 100 % oxygen, there was about 50 per cent inhibition of the rate of CO2‐uptake in continuous and intermittent light compared to that in an atmosphere consisting of 21% oxygen. The same was true of the rate of CO2‐fixation in darkness during a short period after the light was turned off. Since the response to oxygen concentration of the CO2‐uptake in light and of the CO2‐fixation in darkness after the light was turned off were similar, it is concluded that the fixation of CO2 in the short dark period represents an over‐ shoot of photosynthesis. The rate of dark respiration was little affected by the oxygen concentration in the ranges used in the experiments. The carbon dioxide compensation point which has been observed in leaves of 4 to 14‐day old plants was not influenced by either oxygen concentration or light intensity. Since the changes in the rate of CO2‐uptake due to changes in the concentration of oxygen and light intensity had no effect on the CO2‐compensation point, it is concluded that a reabsorption of respiratory CO2 by photosynthesis could not account for the low value of this point. These results are interpreted as a further corroboration of the statement that the leaves of corn lack the process of photorespiration and that dark respiration is inhibited in light.It was observed that the rate of the CO2‐uptake gradually increased in plants which were from 4 to 22‐days old. The inhibitory effect of high concentration of oxygen on the rate of CO2‐uptake was relatively higher in old plants than in young ones.

Transformation of Protochlorophyllide, Formed from Exogenous δ‐Aminolevulinic Acid, in Continuous Light and in Flashlight

Abstractδ‐Aminolevulinic acid supplied to dark grown isolated leaves or wheat causes an accumulation of protochlorophyllide which is only partly transformed to chlorophyllide α in continuous light At the same time a considerable photodestruction of both pigments takes place. By a suitable combinations of short lights flashes and dark periods it is possible, however, to obtain at least double the amount of the protochlorophyllide transformed without photodestruction. The transformation isshown to be dependent on the dark interval between the light flashes. Possible connections with the formation of the protein part of the protochlorophyllide holochrome are discussed.

Control of Flowering of Xanthium pensylvanicum by Red and Far-red Light

A study was made of the effects of various durations, intensities and combinations of red and far-red light interruptions on the flowering responses of Xanthium pensylvanicum Wallr. A dual response to treatments of far-red light was observed. In short dark periods, far-red light alone did not greatly affect flowering but was able to overcome the inhibition of flowering caused by red light. In dark periods longer than 15 hours, far-red inhibited flowering and added to rather than overcame the inhibition by red light. The dark period length required for far-red inhibition remained the same whether far-red was given at the start or at the eighth hour of darkness.In 48-hour dark periods Xanthium showed 3 responses to additions of red and far-red light breaks: A) response to red light; B) response to far-red light; and C) response to red followed by far-red light. Red light given any time in the first 30 hours of darkness overcame the inhibitory effect of far-red light given at either the start or the eighth hour of darkness. Red light given later than the thirtieth hour did not overcome the far-red effect.Approximately the same energy of red light was required to overcome the inhibitory effect of far-red at the second hour of darkness as was required to produce maximum red light inhibition at the eighth hour. Although far-red light was most inhibitory when given early in a long dark period, approximately the same energy of far-red light was required to saturate the far-red response at the fourth, eighth and sixteenth hours.The results are discussed in relation to other reports of far-red inhibition of flowering in short-day plants.

The relation of oxygen evolution to carbon assimilation with isolated chloroplasts

1. In order to determine the relationship to carbon assimilation, the progress of oxygen evolution by isolated chloroplasts was followed using the oxygen electrode. 2. An oxygen uptake was observed in the dark. This was attributable both to the chloroplast preparation and to autoxidation of the medium. 3. In the absence of added Calvin-cycle intermediates the rate of oxygen evolution reached a maximum only after several minutes illumination. 4. In the presence of ribose 5-phosphate the induction period was shortened. In the absence of bicarbonate, evolution came to a stop and could be restarted by the addition of bicarbonate. The oxygen produced corresponded to the CO 2 fixed when the oxygen uptake was allowed for. 5. When illumination of the chloroplasts was interrupted by a short period of darkness, oxygen evolution ceased. When illumination was resumed the induction period was very nearly eliminated. 6. In the presence of substrate amounts of 3-phosphoglycerate no induction period could be observed suggesting that the progress of oxygen evolution is related to the formation of the natural hydrogen acceptor, presumably 1,3-diphosphoglycerate. 7. The results provide direct evidence that there can be a carbon dioxide-dependent oxygen evolution with isolated chloroplasts under aerobic conditions.

Length of Light Period for Maximum Broiler Weight

NUMEROUS studies have been conducted concerning the length of light period necessary for maximum growth in broilers. Clegg and Sanford (1951) found that body weight was increased when broilers were exposed to short periods of intermittent light and darkness compared to when exposed to longer periods. Moore (1957) reported that broilers up to 4 weeks of age grew faster when exposed to continuous light compared to shorter periods. Shutze et al. (1960) indicated that the ability of the chick to grow more rapidly when subjected to continuous light was consistent in all their experiments. Beane et al. (1963) noted a growth advantage in broiler stock subjected to continuous light when compared to those exposed to shorter light regimes. Cherry and Barwick (1962) with small pens where the chicks were given a one week preliminary training found that the subsequent pattern of lighting had no effect on body weight at broiler …

The motion of algae in turbulent flow

AbstractLarge cultures of microscopic, unicellular algae have been proposed as a means of maintaining a life‐supporting atmosphere in a closed, manned system. To achieve vigorous growth of the algal culture it is necessary to subject individual algae alternately to short periods of high intensity light and darkness. One of the means suggested for obtaining a favorable light–dark sequence for photosynthesis is to cause turbulence in a closed channel on which light is incident. Since light is rapidly attenuated in a dense suspension, there will be illuminated regions adjacent to the channel walls and a dark central core. The random motions of turbulence normal to the direction of flow would move the algae alternately from the illuminated regions to the dark region and back again. This paper indicates a method for analyzing the motion of algae into and out of the illuminated region of a channel formed by flat, parallel, transparent plates, with light incident on the plates. Matching of a probability model with a diffusion model makes it possible to estimate the light–dark sequence which could be achieved by turbulence. The results indicate that favorable sequences by this mechanism are unlikely.

THE EFFECTS OF DAYLENGTH ON THE MOVEMENTS OF PULVINATE LEAVES

SummaryVarious illumination cycles, of the sort that are known to have effects in photoperiodism, were tested for effect on the course of the diurnal leaf movements of Bauhinia monandra. Under 12 hour: 12 hour alternations of light and darkness, the leaves open abruptly on illumination and begin to close again rapidly just before the end of the light period. In continued illumination, the evening closure is completed but the leaves reopen at once and further movements are indistinct and irregular. It was concluded that the effects of previous photoperiodic treatment are all but exhausted 12 hours after beginning illumination. Attention was therefore concentrated upon the effects of previous treatments on the timing and amplitude of the evening closure. Moderate lengthening or shortening of the preceding dark period had small, if any, effect on the amplitude, but advanced or retarded (respectively) the time of closure. After very short dark periods (less than 6 hours) closure practically fails altogether. Beginning the illumination period with weak light also delays closure. Light breaks in the dark period advance closure, dark breaks in the daytime retard it, but neither of these effects is evident unless the break is of some hours’ duration. The observations are discussed in relation to Biinning's Tagesrhythmik theory and it is pointed out that the periodicity of Bauhinia leaf movements has been shown not to be constant, whereas, on the other hand, their amplitude is generally more stable. The leaf movements of several other plants have been compared with those of Bauhinia and a classification into long‐ and short‐day types of behaviour as suggested by Bunning has been attempted. This could not, however, be made to agree with their actual photoperiodic sensitivities for flower initiation.

Studies in the nyctinastic movement of the leaf pinnae of Samanea saman (Jacq.) Merrill

Evidence was presented in Part I which demonstrated the presence of an endogenous physiological rhythm in Samanea saman. It was shown that this rhythm controls the nyctinastic movement of the leaf pinna in Samanea saman when the plant is exposed to abnormally long light periods, and correspondingly short dark periods, but has no effect when the plant is exposed to 16 hr. cycles of 8 hr. light/8 hr. dark periods, or 8 hr. cycles of 4 hr. dark/4 hr. light periods. Thus the secondary pulvinus which actuates the leaf pinna shows three responses. Firstly there is the response to darkening of the plant during the course of the light period; secondly there is the response to the endogenous rhythm in continuous darkness or when the plant is ex posed to abnormally long light periods, and thirdly there is the response to abnormally short cycles of light and dark periods. This triple response leads to a consideration of the magnitude and location of the forces which bring about an alternate expansion and shrinkage of the upper and lower halves of the cortex of the pulvinus, which is the cause of the nyctinastic movement. It is recognised that at the cellular level, the movement is brought about by changes in turgor of the cortical cells (Mosebach 1940) and not by any active contraction of protoplasm (Bebt 1867), though how these turgor changes come about is not understood. Using the principle of turgor changes, there are three hypotheses which will explain the nyctinastic curva ture of a secondary pulvinus of the kind that occurs in Phaseolus and Samanea.

Interrelations among Effects of Temperature, Photoperiod, and Dark Period on Floral Initiation of Biloxi Soybean

1. Experiments with Biloxi soybean (Glycine max [L.] Merr.) showed that length of photoperiod as well as length of dark period influenced the degree of flower initiation. With effective dark periods there were lower-critical, optimum, and- upper-critical lengths of photoperiods. With effective photoperiods there were lower-critical, optimum, and upper-critical lengths of dark periods when cycles were shorter than 36 hours long. 2. These critical and optimum lengths of photoperiod became shorter when accompanied by longer dark periods; the converse was also true for critical and optimum lengths of dark period in relation to the length of the accompanying photoperiod. 3. Cycles with photoperiods 8 hours long and cycle lengths ranging from 18 to 56 hours showed two distinct optimum lengths of dark periods. These optima differed in duration by about 24 hours. With relatively short dark periods there was a lower-critical duration below which, and with very long dark periods there was an upper-critical duration beyond which, flowering did not occur. In addition, flowering did not occur at dark periods of intermediate lengths. 4. When these results were plotted on the basis of length of cycle, maximum flowering occurred on cycles either 24 hours long or 48-52 hours long. Therefore, the flowering response in Biloxi soybean is apparently influenced to a marked degree by some endogenous rhythm of 24 hours' duration. 5. Cool temperature during part of the cycle markedly influenced the response of plants to variations in photoperiod, dark period, or length of cycle. When exposed to cool temperature during a portion of cycles of 32-36 hours, the plants flowered. If not exposed to cool temperature in similar cycles, they failed to flower. Cool temperature during a portion of a cycle seemed to have some influence on the endogenous rhythm, so that the length of the rhythmic cycle was other than 24 hours. 6. A discussion is given of the possible nature of this endogenous rhythm and its relationship to the photoperiodic results. Two tentative hypotheses are presented which might explain the results obtained with cycles of different durations at both normal and low temperatures.

PHOTOPERIODIC RESPONSE IN GERMINATION OF SEED OF CERTAIN TREES

Unstratified seeds of Betula pubescens Ehr. and B. verrucosa Ehr. exhibited low or no germination at temperatures from 10° to 20 °C. unless exposed to continuous illumination or illumination interrupted only by a short daily dark period. Germination was high even under a short primary light period if the long dark period was divided into two short ones by an additional daily light period. Besides these true photoperiodic responses, germination was slightly correlated to illumination intensity and to the amount of light. Germination was somewhat reduced when the illumination intensity was either decreased or increased from an optimum. At temperatures of 20° to 25 °C., germination was fair, even under a short daily light period. At still higher temperatures no light was needed for maximum or nearly maximum germination. Seeds of three other tree species also germinated better if exposed to long light periods, but the differences were small or not significant statistically. In most of the experiments the effect of temperature was separated from that of light by using running water as the germination medium. The antibiotic Rimocidin as well as various other chemicals and ultrasonic treatments failed to affect the germination of B. verrucosa.

EMBRYO DEVELOPMENT IN FOUR DATURA SPECIES FOLLOWING SELF AND HYBRID POLLINATIONS

Englislh plantain is a long-day plant developing flowers when exposed to 15 or more long days or to 9 or more days of continuous illumination. Plants have been kept for over 14 months in the vegetative condition by continuous exposure to short-day conditions. Plantain will develop flowers when exposed to 25 long days even though this treatment is not continuous but is interrupted by 10 or 20 consecutive short days. The changes which occur during partial photoperiodic induction apparently persist during relatively long periods of exposure to light and dark periods whose lengths are not favorable for flowering. Plantain exposed to photoperiodic cycles (other than of 24 hr. duration) flowered when exposed to cycles consisting of short light (14 hr. or less) and short dark periods (10 hr. or less), long light and short dark periods, and long light and long dark periods; but remained vegetative when exposed to cycles consisting of short light and long dark periods. It is suggested that in the development of floral structures of this long-day plant, there may be two reactions which occur during exposure to light. The first reaction is unstable and is destroyed during darkness, while the second is relatively permanent and when established will persist during long periods unfavorable to the stimulation of flowering. It is further suggested that the second reaction occurs after the primarv reaction has reached a threshold value, and upon reaching sufficient intensity results in floral development.